The neurotrophins NGF (nerve growth factor), BDNF (brain-derived growth factor), and NT3 (neurotrophin-3) mediate the survival and differentiation of neurons via their specific noncovalent association as homodimers with cell surface receptors, trkA, trkB, and trkC, members of the same family of transmembrane tyrosine kinases. While crystal structures exist for NGF and an extracellular domain of a transmembrane tyrosine kinases (EGF receptor), little is known regarding the structural basis of specificity between these sets of molecules. Such knowledge is needed to develop a molecular therapeutic approach to degenerative diseases of the nervous system.